Casting steel strip

Abstract

In twin roll casting of steel strip, molten steel is introduced into the nip between parallel casting rolls to create a casting pool supported on casting surfaces of the rolls and the rolls are rotated to deliver solidified strip downwardly from the nip. Casting surfaces are textured by a random pattern of discrete projections at least some of which include peaks having a surface distribution of between 5 and 200 projections per mm 2 and an average height of at least 10 microns. The random texture may be produced by grit blasting the casting surfaces on a substrate covered by a protective coating. Alternatively the texture may be produced by chemical deposition or electrodeposition of a coating onto a substrate to form the casting surfaces.

1. A method of continuously casting steel strip comprising the steps of:
 forming casting rolls with textured casting surfaces by grit blasting before a casting campaign is commenced to provide casting surfaces with discrete projections wherein at least some of the projections include peaks having an average surface distribution of between 5 and 200 peaks per mm 2 ;  
 supporting a casting pool of molten steel on one or more said formed textured casting surfaces; and  
 moving the chilled casting surface or surfaces to produce a solidified strip moving away from the casting pool.  
 
     
     
       2. The method as claimed in  claim 1 , wherein the strip is moved away from the casting pool at a speed of more than 40 meters per minute. 
     
     
       3. The method as claimed in  claim 2 , wherein the strip is moved away from the casting pool at a speed of between 50 and 65 meters per minute. 
     
     
       4. The method as claimed in  claim 1 , wherein the molten steel is a low residual steel having a sulphur content of not more than 0.025%. 
     
     
       5. The method as claimed in  claim 1 , wherein a pair of casting rolls are formed with texture surfaces by grit blasting forming a nip between them, the molten steel is introduced into the nip between the casting rolls to create the casting pool supported on the textured surfaces of the rolls immediately above the nip, and the casting rolls are rotated to deliver the solidified strip downwardly from the nip. 
     
     
       6. The method as claimed in  claim 5 , wherein the molten steel is delivered into the nip between the casting rolls via a metal delivery nozzle disposed above the nip. 
     
     
       7. The method as claimed in  claim 1 , wherein each textured casting surface is defined by a grit blasted substrate covered by a protective coating such that the textured pattern shoes in the exterior surface of the protective coating. 
     
     
       8. The method as claimed in  claim 7 , wherein the protective coasting is an electroplated metal coating. 
     
     
       9. The method as claimed in  claim 8 , wherein the substrate is copper and the plated coating is of chromium. 
     
     
       10. The method as claimed in  claim 1 , wherein the textured surface formed by grit blasting is formed of nickel. 
     
     
       11. The method as claimed in  claim 1 , wherein each casting surface is defined by a coating deposited onto a substrate that is then grit blasted to form the casting surface of a random texture. 
     
     
       12. The method as claimed in  claim 11 , wherein the coating is formed by chemical deposition. 
     
     
       13. The method as claimed in  claim 11 , wherein the coating is formed by electrodeposition. 
     
     
       14. The method as claimed in  claim 11 , wherein the coating is formed of a material which has a low affinity for the oxidation products in the molten steel such that the molten steel itself has greater affinity for the coating material and therefore wets the coating in preference to said oxidation products. 
     
     
       15. The method as claimed in  claim 11 , wherein the coating is formed of an alloy of nickel, chromium and molybdenum. 
     
     
       16. The method as claimed in  claim 11 , wherein the coating is formed of an alloy of nickel, molybdenum and cobalt. 
     
     
       17. The method as claimed in  claim 1 , wherein said discrete projections have an average height of at least 10 microns. 
     
     
       18. The method as claimed in  claim 1 , wherein at least some of the discrete projections include peaks having an average surface distribution of between 10 and 100 peaks per mm 2 . 
     
     
       19. The method of  claim 1 , wherein said discrete projections have an average height of at least 20 microns. 
     
     
       20. An apparatus for continuously casting steel strip comprising:
 forming a pair of casting rolls each with textured casting surfaces by grit blasting before the casting campaign is commenced where the casting surfaces have discrete projections wherein at least some of the projections include peaks having an average surface distribution of between 5 and 200 peaks per mm 2 ;  
 assembling the cast rolls with said formed textured casting surfaces into a twin roll caster with the pair of casting rolls horizontally assembled to form a nip between them,  
 a molten steel delivery nozzle for delivery of molten steel into the nip between the casting rolls to form a casting pool of molten steel supported on said textured casting roll surfaces immediately above the nip, and  
 a roll drive that moves the casting rolls in counter-rotational directions to produce a solidified steel strip delivered downwardly from the nip.  
 
     
     
       21. The apparatus as claimed in  claim 20 , wherein the textured casting surfaces of the rolls are each defined by a grit blasted substrate covered by a protective coating. 
     
     
       22. The apparatus as claimed in  claim 21 , wherein the protective coating is an electroplated metal coating. 
     
     
       23. The apparatus as claimed in  claim 22 , wherein the substrate is copper and the plated coating is of chromium. 
     
     
       24. The apparatus as claimed in  claim 20 , wherein the textured casting surfaces of the rolls formed by grit blasting are formed of nickel. 
     
     
       25. The apparatus as claimed in  claim 20 , wherein the casting surfaces of the rolls are each defined by a coating deposited onto a substrate so as to produce a random texture. 
     
     
       26. The apparatus as claimed in  claim 25 , wherein the coating is formed by chemical deposition. 
     
     
       27. The apparatus as claimed in  claim 25 , wherein the coating is formed by electrodeposition. 
     
     
       28. The apparatus as claimed in  claim 25 , wherein the coating is formed of an alloy of a nickel of nickel, chromium and molybdenum. 
     
     
       29. The apparatus as claimed in  claim 25 , wherein the coating is formed of an alloy of nickel, molybdenum and cobalt. 
     
     
       30. The method as claimed in  claim 20 , wherein said discrete projections have an average height of at least 10 microns. 
     
     
       31. The method of  claim 20 , wherein the average height of the discrete projections is at least 20 microns. 
     
     
       32. An apparatus for continuously casting steel strip comprising:
 forming a pair of casting rolls before a casting campaign with the each casting roll having textured casting surfaces by grit blasting with discrete projections at least some of which include peaks having an average surface distribution of between 10 and 100 peaks per mm 2  and an average height of at least 10 microns with the pair of casting rolls horizontally assembled to form a nip between them,  
 a molten steel delivery nozzle for delivery of molten steel into the nip between the casting rolls to form a casting pool of molten steel supported on said textured casting roll surfaces immediately above the nip,  
 and a roll drive that drives the casting rolls in counter-rotational directions to produce a solidified steel strip delivered downwardly from the nip.  
 
     
     
       33. The apparatus of  claim 32 , wherein said discrete projections have an average height of at least 20 microns.